Electromagnetic coils of electrical machinery and apparatus such as transformers are made of covered wires. The covered wires are prepared by coating copper or aluminum wires with insulating resins such as polyimides, polyesters, polyesterimides, polyurethanes, epoxy resins, nylon resins and the like. In order to improve the dielectric strength and the heat resistance of the electromagnetic coils, the coils are coated with epoxy resins, polyester resins or acrylic resins. Such coating is effected by sticking powders of the resin to the heated coils and then fixing the resin to the coils. Alternatively, said coating is effected by allowing to adhere electrostatically charged powders to the coils and then fixed the resin to the coil by fusing the resins.
On referring to the accompanying drawings, FIG. 1 shows a cross section of a coil coated with powdered resin. The numbers of 1, 2, 3 . . . 20 represent covered wires and the order of windings of the wires. The coil is coated with an insulating layer 21 of resin, which is fixed to a surface of the coil. In this coil, voids 22 are formed. Owing to the voids, the insulating layer of coating deteriorates by repeated use of the electrical apparatus, and the dielectric strength thereof deteriorates.
As high voltages are generated in the windings 1, 10, 11, 20 and 5, 6, 15, 16, a high dielectric strength of the coil is required. In the coil coated by the conventional method as stated above, discharge is apt to occur on the windings on account of the voids.
It is an object of the present invention to provide a method of insulating electromagnetic coils by which the coils having high dielectric strength and heat resistance can be obtained.